Helium may soon be used for something a lot more important than balloons. Researchers at the Center for In Vivo Microscopy have developed a technique for taking detailed magnetic resonance images by circulating tiny bubbles of helium throughout the bloodstream, as detailed today in The Proceedings of the National Academy of Science.
Once it makes the leap from mice to human patients, the research may help doctors gain a clearer view of a patient's circulatory system than do existing MRI methods and positron emission tomography scans.
Better pictures allow physicians to focus on clogged arteries or constricted blood vessels. As a result, doctors will be better able to diagnose and treat heart disease and identify organs with a low blood supply.
Pictures created with the technique will have a better signal-to-noise ratio than conventional MRI and, unlike PET scans, the new method will not be hampered by blurry pictures or require exposure to potentially harmful radiation. Project participant Mark Chawla, a graduate student in biomedical engineering, was optimistic about the new technique's imaging potential.
"We see no background [noise] whereas in an MRI image there are protons everywhere," he said, referring to the subatomic particles present in blood and surrounding tissue that add interference to images.
The research extends work done at the Medical Center in 1995, when researchers took helium-3-a form of the element with one neutron-and injected it into a human lung.
An MRI scan picked up the changing magnetic field of the helium and created a highly detailed picture of the lung. However, pure helium cannot be injected into the blood for fear of blocking blood vessels.
The new process begins with hyperpolarized helium-3. Researchers used a two-step process developed at Princeton University to form the atoms they needed. Their first step involved heating a vessel filled with helium and rubidium to 150 degrees centigrade and exposing it to laser light. The laser light changes the direction in which rubidium electors spin, and as the tiny particles collided with the much larger helium nuclei the desired spin was passed along.
The hyperpolarized helium is then suspended in a contrast agent through hand agitation-"shaking" in layman's terms-and injected into the bloodstream. A magnetic field generated by MRI causes all the helium nuclei to align on one plane, some pointing "up" and others pointing "down." Then, researchers use a radio frequency pulse to drive the nuclei perpendicular to their original plane. The changing magnetic field caused by the nuclei's movement induces an electric current that can be measured.
The benefits of this mechanism could be far-reaching. Currently, PET scanning is used to visualize blood vessels. However, this requires radioactive dye which can damage body cells. Since the dye circulates through the body, it disperses rapidly over time, blurring the image obtained. In contrast, hyperpolarized helium is depolarized by MRI as it is detected, preventing the signal from diffusing and coarsening.
Center director Dr. Allan Johnson and his team placed helium in an ultrasound suspension, forming microbubbles with diameters of between two and 30 microns. Researchers believe that these microbubbles can travel safely in the blood stream because they are similar in diameter to blood cells.
Dr. James Brookeman of the University of Virginia said the new technique would be kinder to patients, avoiding exposure to radiation or invasive procedures. He explained that the old system of viewing the heart involved running a catheter into the heart, and using iodine to make X-rays stand out clearly.
The new system also eliminates the problem of iodine leakage into surrounding tissue.
This current study is only a feasibility study. Johnson estimated it will take two years to optimize the technique, which will then be adapted commercially pending approval by the Food and Drug Administration. However, FDA approval occurs in "an uncertain time frame at best," Johnson said.
The research was supported by National Institutes of Health, the National Center for Research Resources, the National Science Foundation and the Whitaker Foundation.
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